The present invention relates generally to lasers, and more particularly to a method for compressing laser pulses and, further, for suppressing prepulse laser emissions.
Increasingly, in the effort to develop new and better uses for lasers, it has been found to be necessary or desirable to manipulate the character of laser beams and pulses. Accordingly, means have been developed to lengthen, shorten, intensify and/or modify in form the output of pulsed lasers. Furthermore, improvements to such means, and new and better means for accomplishing these purposes, continue to be much sought after.
One of the purposes to which the lasers are just now beginning to be applied is the stimulation of X-ray radiation. The present inventors are engaged in utilizing such stimulated X-ray emission for applications such as Xray lithography. In this application the laser beam is directed at a metal or metal oxide target which is excited thereby to emit X-ray radiation. It is necessary to have a relatively high power laser pulse for this purpose, and it is also important that the power of the pulse be confined in time to a relatively short duration and that there not be a "prepulse" emission (an unwanted laser emission preceding the desired high power pulse), for reasons which will be discussed hereinafter.
In order to achieve a pulse having the desired short duration and high power characteristics, methods of pulse compression have been tried. Pulse compression is desirable as a means for achieving this goal, since modulation of the laser excitation to achieve high power pulses of short duration is difficult and, further, since the required instantaneous power may be greater than that which the laser emitter is capable of producing. Pulse compression is of particular interest to those interested in stimulated X-ray emission because experimental evidence indicates that X-rays are generated with much greater efficiency when the laser energy is delivered within the time scale of one to several nanoseconds, rather than in a longer time such as ten to twenty nanoseconds. However, operation of the laser with such high energy short pulses places a much greater load on the laser cavity optics which may be damaged by intense laser radiation. Consequently, it is desirable to have a means for allowing long pulses of moderate peak energy within the laser amplifier and then to externally shorten these pulses for more efficiency in generating the X-rays.
It is known in the art to use the phenomenon of stimulated Brillouin scattering ("SBS") to provide pulse modification. U.S. Pat. No. 3,617,927 teaches an arrangement of components for producing "giant" pulse using SBS. However, to the inventors' knowledge, no means for using SBS to produce a very high power output laser pulse is known in the prior art. Prior art means for producing a compressed pulse all require that a large amount of energy of the long pulse be focused into the SBS cell, thus limiting the total amount of energy that can be compressed.
As mentioned previously, another consideration in the application of lasers for the stimulation of X-ray emission is that the "prepulse" should be suppressed. Prepulse suppression is desirable because, in high power laser systems such as the regenerative amplifier under development at Lawrence Livermore National Laboratory, a pulse as large as 1% to 5% of the energy of the main pulse can be generated several hundred nanoseconds before the desired output. It would be desirable to implement a technique for suppressing these prepulses so that they do not arrive at and deplete the target prior to the arrival of the main pulse. To the inventors' knowledge, no method for accomplishing this that does not require active electro-optic switches (Pockels cells) has existed in the prior art.
No prior art means, to the inventor's knowledge has provided highly efficient pulse compression of a high power laser pulse. Nor has any prior art means adequately suppressed prepulse emissions of high power lasers. All previous means for compressing laser pulses have been incapable of compressing high energy pulses.